Emulsion polymerization involves the use of a suitable surfactant or soap for polymerizing a water-insoluble monomer or monomers in an aqueous dispersion. The surfactant or soap generates micelles which are the actual polymerization loci. As the polymerization proceeds, the micelle grows by imbibing monomer or monomers and ultimately yields a microscopic polymer particle. The final product is an aggregate of these particles which is a milky latex that contains 20-40% solids. The latex may be used directly, as in latex paint, or can be coagulated to recover the dry polymer.
Emulsion polymerization may be used to produce very high molecular weight polymers by free radical mechanism, as is widely practiced by and commercially important to the rubber industry. The use of water as a diluent and heat transfer medium is very advantageous in emulsion polymerization processes.
The primary disadvantage of emulsion polymerization techniques is that a high concentration (up to 6 weight percent) of emulsifier residue inherently remains in the coagulated polymer.
It is thus advantageous to employ an emulsifier system which is compatible with prior processing, assembly, and vulcanization operations. Rosin and fatty acid emulsifiers are the materials of choice because of their secondary advantages in rubber processing. Rosin is a necessary ingredient to improve the rubber's milling characteristics.
The unmodified rosin contains 30-50 wt % of abietic acid isomer which is known to retard the polymerization reaction and produces unwanted polymer structures. It is desirable to have less than 1 wt % abietic acid in the modified rosin in order to produce a quality emulsifier. The chemical reaction which produces emulsifier-grade rosin is known as "disproportionation of rosin." In the disproportionation of rosin, most of the abietic acid is converted to dehydroabietic acid by a dehydrogenation reaction.
The disproportionation of rosin is generally carried out with a suitable catalyst at an elevated temperature. A palladium on carbon catalyst has been used for disproportionation, but is unsuitable for disproportionation of tall oils due to sulfur impurities which poison the catalyst. Other catalyst systems include organic phenol sulfide, iodine, and metal iodides with and without iron for producing disproportionated tall oil based rosin for emulsifiers. For example, U.S. Pat. No. 4,271,066 to Matsuo et al. teaches a process for disproportionating rosin in the presence of a catalyst comprising (1) at least one iodide selected from the group consisting of sodium iodide and potassium iodide, and (2) at least one iron compound selected from the group consisting of iron, iron oxides, iron hydroxides and iron sulfides.
The production of liquid rosin esters using a thermal reaction between rosin and an alcohol in the presence of anthraquinone and at least one alkali metal iodide is described in U.S. Pat. No. 4,822,526 to Tsuchida et al. The alkali metal iodide is either sodium iodide or potassium iodide. Other related art includes U.S. Pat. No. 4,481,145 to Timms, which discloses a method for disproportionating rosin or tall oil by heating the rosin or tall oil with a catalyst comprising iodine and an iron compound using ammonia, an ammonium salt, or an amine as an additional component of the catalyst. The method described in U.S. Pat. No. 4,659,513 to Correia is similar to that of Timms but includes a pretreatment step by which the starting material is heated with an effective amount of elemental sulphur.
A process for treating tall oil fatty acids with a bromine-iodine catalyst mixture to convert the linoleic acid portion to oleic acid and other fatty acids is set forth in U.S. Pat. No. 3,860,569 to Ward. The source of the iodine catalyst may be as free iodine or such iodine sources as amine hydroiodides, aliphatic organic iodides and inorganic iodides, such as aluminum iodides.
U.S. Pat. No. 5,023,319 to Hollis et al. discloses a method for treating polymerized rosin to decrease its tendency to oxidize comprising heating the rosin to an elevated temperature in the presence of a catalytic amount of any known disproportionating agents such as iodine. Other patents include U.S. Pat. No. 5,175,250 to Hazen (producing a stabilized rosin by heating the rosin in the presence of a phosphoric acid or a strong acid in combination with a phosphate-containing substance and disproportionating the rosin by the addition of iodine); U.S. Pat. No. 4,788,009 to Johnson, Jr. (preparing a polyol ester of rosin, which comprises esterifying the rosin with the polyol in the presence of a catalytic proportion of a compound selected from the group consisting of alkali metal salts and alkaline earth metal salts of either phosphorous acid or hypophosphorous acid); U.S. Pat. No. 1,131,939 to Melamid (manufacturing pure resin-oil by heating a mixture of resin and phosphoric acid); and U.S. Pat. No. 3,544,474 to Shaffer (soap composition comprising a dry mixture of an alkali metal soap of a disproportionated rosin distended on a finely-divided microporous hydrous calcium silicate).